The present invention relates in general to the purification of gases, and, in particular, to the concentration of methane from a gas feed stream.
Methane comes from a variety of reactions. Methane is a good fuel gas, it being the major constituent of natural gas. Examples of sources of methane gas include biological degradation of sewage waste or digester gas, foodstuff waste, animal feed lot waste, and land fills. Interest in these sources of methane gas has heightened because of the energy crisis.
Typical gaseous products from these sources are heavily contaminated with carbon dioxide and hydrogen sulfide. Hydrogen sulfide must be removed because of its toxicity and odor. Carbon dioxide reduces the heating value of the gas and it is desirable to remove it for that reason. In some applications the value of the gas must be comparatively high to merit the gas's use.
One proposed use for concentrated methane from these sources is as a fuel for automobiles. The gas is stored in high pressure storage tanks, at a pressure of up to 3,600 p.s.i.g. The stored gas is then transferred to high pressure storage tanks of vehicles to pressures up to about 3,000 p.s.i.g. Vehicles can be filled directly, bypassing storage.
Another proposed use of the methane is as pipeline gas. The reclaimed methane of adequate quality is introduced into utility provided natural gas to augment that supply.
It is known that carbon dioxide and hydrogen sulfide can be absorded from methane by passing the stream containing the three gases countercurrent to water. The water absorbs the carbon dioxide and hydrogen sulfide. It is also known that the residence time of the gas stream in contact with the water and the quantity of water affect resulting purity; the more water, the higher the effluent stream's purity; the greater the residence time, the greater the purity. It is also known that residence time can be increased by increasing the height of a column used for the counterflowing process streams. Further, it is known that increases in pressure enhance purity by making the impurities more soluble in water.
It is also known that water containing hydrogen sulfide and carbon dioxide can be freed of those contaminants by stripping with air. Iron sponge reactions where hydrogen sulfide is oxidized to elemental sulfur and water is also known.
To purify streams containing methane, the process should be efficient, reliable, and versatile.